Breast cancer most commonly metastasizes to the skeleton and improvements in therapy for osteolytic bone metastases are required. Oncdlytic adenoviruses tailored to replicate selectively with tumor cells are novel anticancer agents with great therapeutic potential but have shown limited efficacy in the clinical setting. We have demonstrated that efficient oncolysis by a replicating adenovirus is critically dependent on tumor expression of primary adenovirus receptors. A number of studies that have shown that primary cancer cells express low levels of the coxsackievirus and adenovirus receptor, CAR,and are poorly infected by adenoviruses. Moreover, the widespread distribution of CAR on noncancerous tissues presents an obstacle to the selective delivery of replicating adenoviruses to disseminated breast cancer cells upon systemic administration. These two concerns imply that modification of a replication-selective adenovirus to allow efficient and selective CAR-independent infection of target cancer cells could both improve the efficacy of the virus and reduce toxicity to normal cells. In addition, the efficacy of a replication-selective adenovirus could be enhanced by engineering it to deliver a therapeutic transgene. We propose to arm the replicating adenovirus with a secreted therapeutic protein, with a distinct mechanism of action within the local bone microenvironment - osteoprotegerin (OPG), which inhibits bone resorption. We hypothesize that a replication-selective adenovirus armed with OPG would eradicate bone metastases of breast cancer both directly, by oncolysis, and indirectly, by inhibiting osteoclastic bone resorption and thus reducing the tumor burden. Taken together, we propose to combine two distinct strategies to improve the efficacy of replicating adenoviruses for the treatment of bone metastases of breast cancer. The first specific aim is to construct a tropism-modified, armed replicating adenovirus expressing OPG. The second specific aim is to evaluate the efficacy of the tropism-modified, armed replicating adenovirus in vitro. The third specific aim is to evaluate the efficacy of the tropism-modified, armed replicating adenovirus in vivo. This will establish the therapeutic potential of this novel agent for the treatment of bone metastases of breast cancer in humans.